Mainly a bimetal, a PTC thermistor, etc. are known as over-current protection elements to be used as a counter measure an overload of a small-sized DC motor, and there is a tendency that those elements are particularly used in the automobile industry.
Of those elements, a bimetal is connected in series mainly to a motor having a diameter of 40 mm or more, that is, having an overload current of about 5 A or more, and usually used as the motor built-in type.
Generally, when an overload current is not larger than 5 A, the accuracy of switching operation of a bimetal is low so that the bimetal fails to perform stable operation. Accordingly, a PTC thermistor having a cost lower than that of the bimetal is broadly used.
As is known, a PTC thermistor is a resistor element having a positive temperature coefficient, and although the resistance value thereof is low at an ordinary temperature, the value suddenly increases to be 10.sup.4 -10.sup.7 times as large as the value at an ordinary temperature if the element reaches or exceeds a predetermined switching temperature because of self heating due to an over-current or heat transfer from a certain heat source.
A PTC thermistor made of BaTiO.sub.3 ceramics is most generally known, and a PTC thermistor of V.sub.2 O.sub.3 ceramic, a resin PTC thermistor in which conductive particles such as carbon black or the like are contained in polyolefin resin, and the like, are known in addition to the BaTiO.sub.3 PTC thermistor.
Conventionally, a PTC thermistor to be used in a small-sized DC motor is made of semiconductor ceramics containing BaTiO.sub.3 as the principal component. Because of limitation in the material characteristic that the resistivity of the material must be not lower than 8 .OMEGA.cm and the withstand voltage be in a range of 30-50 V, in use, a PTC thermistor is usually connected in series to a motor as a disk-like part provided with a lead wire having a diameter not thinner than 10-12 mm. Since a PTC thermistor cannot be incorporated into a motor because of its shape and its size, however, it is general that a separate PTC thermistor is mounted on the outside a motor through a printed wiring board or the like.
Consequently, there has been a problem that it takes a long time to mount a PTC thermistor, and development of a small-sized motor, even a small-sized motor having a diameter smaller than 40 mm, provided with an overload protection element is desired by a car industrial field, etc., who seek reduction in size and in weight.
In such a background, the inventors of this application previously proposed "A small-sized DC motor" (Japanese Patent Application No. Hei-1-292397) and "A small-sized DC motor (Japanese Patent Application No. Hei-1-292398). They are those structured fundamentally with materials having characteristics considerably improved than those of the conventional materials. Next, those motors will be described.
The first small-sized DC motor disclosed in Japanese Patent Application No. Hei-1-292397 is characterized in that a BaTiO.sub.3 ceramic in which the ratio (V.sub.B /.rho..sub.25) of the withstand voltage V.sub.B (V) to an ordinary-temperature resistivity .rho..sub.25 (.OMEGA.cm) is not smaller than 7 and not larger than 20 is connected in series to an armature winding and the PTC thermistor is incorporated in a motor casing. The BaTiO.sub.3 ceramic is formed of, as starting raw materials, a mixed crystal or a solid solution of BaTiO.sub.3, SrTiO.sub.3, PbTiO.sub.3, and CaTiO.sub.3 synthesized through a liquid-phase solution reaction method, and a valency control agent and Mn and Si which are added to the above principal components by predetermined amounts. Further, as the arrangement position of the PTC thermistor made of the foregoing materials, places such as a position between a phosphor-bronze plate connected to a brush of the small-sized DC motor and input terminals, a position between the brush and the phosphor-bronze plate connected to the input terminals, etc. have been proposed.
The small-sized DC motor disclosed in Japanese Patent Application No. Hei-1-292398, on the other hand, is characterized in that a resin PTC thermistor is connected in series to an armature winding and incorporated in a casing of the motor. The material of the PTC thermistor a mixture of polyolefin or halogen-insulating resin and a high conductive material of a carbon group, the mixture generally having ordinary-temperature resistivity of 1-2 .OMEGA.cm and withstand voltage not lower than 100 V. Further, as the position where the PTC thermistor made of the foregoing material is to be provided, a position between a phosphor-bronze plate connected to an input terminal and a brush of the small-sized Dc motor, a position between the brush and the phosphor-bronze plate connected to the input terminal, etc., have been proposed.
Both the above small-sized DC motors are characterized in that the PTC thermistor is formed into a small-sized or thin plate-like chip or into a cylindrical shape or a shape according the foregoing shape and disposed in the inside of the motor, on the basis of the fact that the material characteristic of the PTC thermistor, particularly, the withstand voltage and the ordinary-temperature resistivity have been considerably improved.
Each of the above proposed small-sized DC motors, however, has the following problems, while the PTC thermistor superior in material characteristic is formed like a chip and incorporated into the motor so as to make the motor have an overload protecting function itself and be reduced in size as well as in weight/:
(1) With respect to the position where the PTC thermistor is incorporated in the small-sized DC motor and the shape of the PTC thermistor, a plate-like PTC thermistor having electrodes at its opposite surfaces and being arranged between the input terminal and the phosphor-bronze plate connected to the brush, a PTC thermistor having electrodes at its opposite surfaces and being arranged between the input terminal and the phosphor-bronze plate connected to the brush, a cylindrical PTC thermistor having electrodes at its opposite end surfaces and being arranged on a terminal board, etc. have been proposed. In each of the cases, there is a possibility of occurrence of problems that the space for position of arrangement is very small so that the size of the chip-shaped PTC thermistor is limited because of the foregoing limitation of the space, the mounting is not easy, the PTC thermistor may be allowed to come off or damaged by vibrations and/or shocks. PA1 (2) In a motor having internal resistance not larger than about 4 .OMEGA. even if the motor has a diameter not larger than 40 mm, the PTC thermistor which matches with the motor must have a resistance value not larger than 1 .OMEGA.. Assuming that such a PTC thermistor is formed by using the materials proposed in Japanese Patent Application No. Hei-1-292397, then the size of the thermistor is not smaller than about 5 mm.times.5 mm in the case of the square chip, and in order to mount the PTC thermistor in the casing with no problem, a considerable device is required in view of space and vibration-resistance. PA1 (3) Although detailed description will be made later with respective to the embodiment, a PTC thermistor is a heating resistor, and the resistance value of the thermistor under a predetermined condition is determined by a balance between the heat generation of the thermistor itself owing to Joule heat and the heat radiation to the circumference. That is, the resistance value relates to not only the shape of the thermistor and the circumferential heat environment but the heat capacity of the thermistor, that is, the size thereof. In other words, this fact means that the smaller the size of the thermistor becomes the smaller the heat capacity thereof becomes so that thermistor becomes more sensitive against a circumferential influence. PA1 (4) The fundamental point of the foregoing proposals is in the material in which the ratio of the withstand voltage to the ordinary-temperature resistivity is remarkably improved in comparison with the conventional one. The material is a BaTiO.sub.3 PTC ceramic synthesized through the liquid-phase solution reaction method, or PTC resin. Those materials are higher in cost than BaTiO.sub.3 ceramic prepared by the conventional solid phase method, and therefore such a PTC thermistor that can be incorporated in a motor even if it is made of the conventional material has been desired.
Consequently, in the case where a thermistor is merely decreased in size by using a material having a large ratio of withstand voltage to resistivity at ordinary temperatures and is incorporated in a motor as shown in the preceding proposal, when an overloaded state occurs in the motor, heat generation of a motor coil is transferred to the PTC thermistor through a phosphor-bronze plate or the like by the heat coupling effect so that the temperature of the PTC thermistor is raised easily and there is an advantage that the response of the switching operation of the PTC thermistor is made high, and in the current limiting state, on the contrary, the withstand voltage is made high as if a radiation plate is provided. There is, however, a defect in that the Joule heat of the PTC thermistor is transferred to the motor to thereby increase the limitation current value. Consequently, if the PTC thermistor is merely reduced in size, a bad influence is rather generated in view of heat capacity.